Furnace and process for providing a source of molten metal

ABSTRACT

A furnace for providing a source of molten aluminum or other metal includes a main chamber and a sweat chamber, the floor of which constitutes a sweat hearth on which ingots or sows are placed. A division wall separates the sweat chamber from the main chamber, and this wall contains ports located at the level of sweat hearth for providing communication between the two chambers. Each chamber, moreover, contains its own set of burners and has a discharge stack leading away from it. Each stack in turn has a refractory damper in it. The burners within the main chamber maintain the bath in a molten condition, and when these burners are in operation, the damper in the stack leading from the main chamber is closed, while the damper in the stack leading from the sweat chamber is open. Thus, the hot gases flow through the ports in the division wall and thence through the sweat chamber to the stack leading from that chamber. The hot gases preheat the sows. The burners in the sweat chamber are only operated after the main burners are shut off and the dampers reversed opened. The sweat burners melt the sows, and the molten metal flows into the bath through the ports in the division wall. The hot gases again flow through the division wall, but in the reverse direction, and thus pass through the main chamber to the stack leading from that chamber, through which they are vented. The gases thus maintain the bath in a molten condition.

BACKGROUND OF THE INVENTION

This invention relates in general to furnaces and more particularly to afurnace and process for melting metals so as to provide a source ofmolten metal.

Aluminum and its alloys melt at relatively low temperatures, and thiscoupled with other desirable characteristics, render aluminum ideal forcasting and extruding operations. It is quite common to derive themolten aluminum for these operations from a furnace into which bothaluminum ingots or sows and aluminum scrap are introduced. The sows,however, require special precautions in handling, because they may, ifintroduced into a bath of molten aluminum produce an explosion. In thisregard, the typical cast aluminum sow often contains shrinkage cracks inwhich water collects, and this water if suddenly elevated intemperature, such as might occur if the sow were deposited into the bathof molten aluminum, could well produce an explosion of sufficientmagnitude to destroy the furnace.

For this reason it is common to slowly preheat aluminum sows so a tovaporize the water and drive it off before the sows are introduced intothe molten aluminum. Such preheating may take place within the meltingfurnace itself or in a separate preheating furnace.

In this regard, the typical aluminum furnace has a sill located to theside of the molten aluminum bath and exposed in its entirety to theheated chamber over the bath. This chamber is heated by burners whichare directed into it, and these burners supply enough heat to melt thealuminum and maintain it in a molten condition. The sows to the side ofthe bath absorb much heat from the chamber, both through the effects ofradiation and convection, and indeed the latter is enhanced by ventingthe chamber through the region in which the sows are located, so thatthe heated gases flow across the sows as they leave the chamber. In timethe sows melt and molten aluminum which is produced flows into the bath,thus adding to the bath.

The presence of a large number of sows in the chamber enables the sowsto absorb heat which might otherwise be directed into the aluminum bathto maintain the aluminum of that bath molten. In other words, the wallsof the melting chamber, instead of radiating heat almost entirely intothe bath, radiate a substantial amount of heat to the sows. This wastesenergy in that the chamber must be overheated to effect a melting of thesows.

SUMMARY OF THE INVENTION

One of the principal objects of the present invention is to provide afurnace and process that is ideally suited for melting metals, includingaluminum sows. Another object is to provide a furnace and process of thetype stated in which the molten bath of metal is on all sides exposed tochamber walls which radiate heat back into the chamber and to the bathcontained in it. A further object is to provide a furnace and process ofthe type stated which is highly efficient. An additional object is toprovide a furnace and process of the type stated in which sows areheated and melted in a chamber separate from the chamber to which thebath of molten metal is contained, with each chamber having its ownburners, and when one chamber is heated the hot gases from it flowthrough the other chamber to provide heat for that other chamber. Stillanother object is to provide a furnace of the type stated in which thechamber where the sows are melted may be heated with burners that aredirected into that chamber. Yet another object is to provide a furnacewhich may be loaded without exposing the high temperature meltingchamber of the furnace. Still another object is to provide a furnace andprocess which uses two chambers, each having its own burners, own flueand damper for such flue, so that hot gases produced by the burners ofone chamber may be directed through the other chamber to supply heat tothat other chamber.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings which form part of the specification andwherein like numerals and letters refer to like parts wherever theyoccur:

FIG. 1 is a perspective view of a furnace constructed in accordance withand embodying the present invention, the furnace being partially brokenaway and in section at its sweat hearth;

FIG. 2 is an elevational sectional view of the furnace;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 2 and showingthe main chamber burners and the discharge stacks in phantom lines;

FIG. 4 is an elevational sectional view taken along line 4--4 of FIG. 3and showing the main chamber;

FIG. 5 is an elevational sectional view taken along line 5--5 of FIG. 4and showing the sweat chamber; and

FIG. 6 is a sectional view taken along line 6--6 of FIG. 2 and showingthe reaction chamber for the flue system.

DETAILED DESCRIPTION

Referring now to the drawings, a furnace A (FIGS. 1 & 2) serves as asource of molten aluminum for a manufacturing operation such as castingthe aluminum into configurations dictated by the shape of a mold orextruding into shapes determined by an extrusion die. To this end, thefurnace A contains a bath B of molten aluminum, and this molten aluminummay be derived from scrap aluminum products, such as expended beveragecans, or it may be derived from aluminum sows C (FIGS. 3 & 5) which areingots that are normally obtained from a refiner of aluminum ore.

The bath B is for the most part contained in a primary or main chamber 2(FIG. 2), although some of it is in a pumping well 4 (FIG. 3) and stillmore is in a charging well 6, both of which are located to one side ofthe main chamber 2. Actually, the main chamber 2, pumping well 4 andcharging well 6 are all connected such that molten aluminum will flowfrom the main chamber 2 into the pumping well 4 to thereafter circulatethrough the charging well 6 and back into the main chamber 2, but theupper surface of the bath B remains at the same elevation in all three.The sows C, on the other hand, are placed in a sweat chamber 8 which islocated to the other side of the main chamber 2. The main chamber 2, thetwo wells 4 and 6, and the sweat chamber 8 are enclosed by various wallswhich are for the most part formed from a refractory material, such asrefractory brick. Finally, the furnace A includes a flue system 10 whichreceives heated gases and products of combustion from the main chamber 2and sweat chamber 8, as well as volatilized substances from chargingwell 6.

Among the walls which enclose the main chamber 2 (FIGS. 2-4) are a pairof spaced apart side walls 16 and 18 and an end wall 20 which connectsthe side walls 16 and 18 at one end of the chamber 2. The other end ofthe chamber 2 is closed by doors 22 (FIG. 4) which, like the walls 16,18 and 20, are formed from a refractory material and are for all intentsand purposes walls themselves. However, the doors 22 may be raised witha hoist mechanism 23 to expose the main chamber 2. The side walls 16 and18 and the end wall 20 extend downwardly to a floor 24 as well asupwardly to a roof 26, both of which are flat and generally horizontal.However, the floor 24 at one end of the chamber 2 has a ramp 28 thatleads up to the doors 22. Actually, the ramp 28 leads up to a sill 30which in turn extends outwardly toward the lower ends of the doors 22.While the sill 30 is higher than the upper surface of the molten bath B,the bath covers most of the ramp 28. Thus, the bath B is confined at oneend by the end wall 20, at its opposite end by the ramp 28, and alongits sides by the side walls 16 and 18.

The end wall contains a tap 31 (FIG. 4) through which molten aluminummay be withdrawn from the furnace A on a continuous or intermittentbasis.

The roof 26 contains several burners 32 (FIGS. 2 & 4--shown in phantomlines on FIG. 3) which are directed downwardly into the main chamber 2and produce enough heat at a temperature high enough to maintain thealuminum that comprises the bath B in a molten condition. Since theupper portion of main chamber 2, that is the portion which is notoccupied by the bath B, is confined on all four sides by refractorywalls, or more particularly on its sides by the side walls 16 and 18 andon its ends by the wall 20 and doors 22, the burners 32 not only directheat into the metal bath B, but also into the surrounding walls 16, 18,20, and 22 which in turn reradiate the heat into the bath B, and thesame holds true with regard to the roof 26. In this regard, the spacebetween the roof 26 and the upper surface of the bath B exceeds thedepth of the bath B, so that a relatively large surface area existsaround and over the bath B. This almost total confinement of the mainchamber 2 provides maximum heating efficiency.

The sweat chamber 8 (FIGS. 2, 3 & 5) exists on the opposite side of theside wall 16 so the side wall 16 is actually a division wall whichseparates the two chambers 2 and 8. At its bottom the sweat chamber 8has a hearth 34 which slopes downwardly at a slight inclination towardthe side wall 16 and the main chamber 2, its lower margin being slightlyhigher than the upper surface of the bath B. Indeed, the side wall 16near its ends is provided with ports 36, the lower surfaces of which areflush with the upper surface of the sweat hearth 34. Here the hearth 34has its lowest elevation, so that any metal which melts in the sweatchamber 8 flows downwardly across the sweat hearth 34 and thence throughthe ports 36 into the bath B to add to the metal of the bath B. In termsof cross-sectional area, the ports 36 are just large enough toaccommodate flue gases from the main chamber 2.

The sweat hearth 34 (FIGS. 2, 3 & 5) forms the floor of the sweatchamber 8 and is located opposite a roof 38 which is at about the sameelevation as the roof 26 of the main chamber 2. The sweat chamber 8 islikewise enclosed by refractory walls, one of these walls being the sidewall 16 which separates the sweat chamber 8 from the melting chamber 2.Another, is the end wall 20 which extends laterally beyond the side wall16 to form one end of the sweat chamber 8. Still another is an end wall40 which is located at the opposite end of the sweat chamber 8 andextends laterally from the side wall 16. The remaining side of the sweatchamber 8, that is the side located opposite to the side wall 16, isclosed by a pair of doors 42 (FIGS. 1 & 2) having upwardly extendinghangers 44 which normally loop over brackets 46 that are set above theroof 38 of the sweat chamber 8. When the hangers 44 rest on the brackets40, the doors 42 completely close the side of the sweat chamber 34, butthe doors 42 are attached to a hoist mechanism 48 which when activated,elevates the doors 34 enough to expose the sweat chamber 8, so that thealuminum sows C may be placed on the sweat hearth 34 to be thereaftermelted.

The end walls 20 and 40 contain burners 50 (FIGS. 3 & 5) which aredirected into the sweat chamber 8 to provide a flame which suppliesenough heat at a temperature high enough to melt aluminum sows C thatare on the sweat hearth 34. Of course, the flames from the burners 50not only heat the sows directly, but also heat the walls 16, 20 and 40,the doors 42, the hearth 34, and the roof 38, all of which are formedfrom refractory material, and these surfaces reradiate heat to the sowsC to facilitate their conversion to a liquid state. Thus, the sweatchamber 8 likewise avails itself of a most efficient heating principle.

The pumping well 4 (FIG. 3), which is located on the opposite side ofthe melting chamber 2 from sweat chamber 8, is rectangular inconfiguration, it being closed on one side by the side wall 18 and onits other side by a short wall 54. One end of the pumping well 4 isclosed by the end wall 20 which extends beyond the side wall 18, whilethe opposite end is closed by a division wall 56 which extends betweenthe side wall 18 and the outside wall 54. Moreover, the pumping well 4has a refractory floor which is at the same elevation as the floor 24 ofthe main chamber 2, and indeed the main chamber 2 and pumping well 4 arein communication through an opening 60 in the side wall 18, that openingbeing submerged in the bath B so the molten aluminum will flow from themain chamber 2 into the pumping well 4. The division wall 56 on theother hand has an opening 62 which provides communication between thepumping well 4 and the charging well 6, and that opening is likewisesubmerged in the bath B. Located in the pumping well 4 is a pump 64which draws molten aluminum from the main chamber 2 through the opening60 in the side wall 18 and into the pumping well 4 and furtherdischarges that molten aluminum from the pumping well 4 through theopening 62 in the separating wall so that it passes into the chargingwell 6. The walls 18, 20, 54, and 56 which surround the pumping well 4support a cover 66 (FIG. 1) which extends over the well 4 to containheat within it. However, the cover 66 may be removed to service the pump4.

The charging well 6 (FIGS. 3 & 5) is likewise rectangular, it beingclosed on one side by the side wall 18 and on another by the divisionwall 56. Along its bottom is a floor 70 which is at the same elevationas, or in other words flush with, the floor 24 of the main chamber 2.The two other sides of the charging well 6 are enclosed by relativelysteep ramps 72 which extend upwardly from the floor 70 at an angle ofabout 45° with respect to the floor 70 and at the surface of the bath Bmerge into dross ramps 74 of lesser pitch.

The side wall 18 near the floors 24 and 70 of the main chamber 2 andcharging well 6 has another opening 76 (FIG. 3) which providescommunication between the well 6 and chamber 2, and this permits moltenaluminum to flow from the charging well 6 back into the bath B in themain chamber 2. Indeed, the pump 64 circulates the molten aluminumthrough the charging well 6, causing it to flow into the well 6 at theopening 62 in the separating wall 56 and out of the well 6 at theopening 76 in the side wall 18.

Extended over the charging well 6 is a fume hood 80 (FIGS. 1 & 2) havingopenings at the outer margins of the dross ramps 74, and these openingsare normally closed by doors 82 which are connected to hoist-typeelevating mechanisms 86. When either of the doors 82 is elevated, thecharging well 6 is exposed through the opening normally covered by thatdoor 80. Thus, with the door 80 elevated, a charge of scrap aluminum,such as expended beverage cans, aluminum turnings, or aluminum sheetmetal, may be introduced into the portion of the bath B contained withinthe charging well 6. Paints, grease and similar substances on the scrapimmediately volatilize and collect within the fume hood 76. Of course,the scrap eventually melts within the charging well 6 to become part ofthe bath B. Also, when either door 80 is open, an attendant may rake thedross that collects in the charging well 6 upwardly onto and over thedross ramps 74 at that door 80.

The flue system 10 includes (FIG. 2) two discharge stacks 90 and 92,each of which at its upper end opens into a connecting pipe 94 which inturn opens into a fume reactor 96 that is located over the roof 26 ofthe main chamber 2, but is not connected directly to the main chamber 2.Indeed, the reactor 96 communicates with the main chamber 2 only throughthe discharge stack 90. That stack, which is rectangular incross-sectional configuration, is lined with refractory material thathas a ledge or damper seat 98 intermediate its ends and an opening 100adjacent to that seat. Indeed, the sides of the seat 98 are inclineddownwardly to the lower margin of the opening 100 where a refractorydamper 102 is hinged to the stack 94. The damper 102 possesses a flatclosure wall 104 and an arcuate upper wall 106, the latter beingconcentric with respect to the hinge axis for the damper 102. Both arelined with refractory material. Like any damper, the damper 102 movesbetween open and closed positions, but irrespective of its position, thearcuate wall lies along the upper margin of the opening 100. When thedamper 102 is closed, the flat closure wall 104 at its periphery restson the damper seat 98, forming a reasonably good seal with it so as toblock the flow of hot gases from the melting chamber 2. On the otherhand, when the damper 102 is open, the flat closure 104 lies generallywithin the opening 100, and while the damper 102 permits hot gases torise through the discharge stack 94, it nevertheless prevents them fromescaping at the opening 100. Thus, the hot gases from the main chamber 2are directed into the connecting pipe 96 and thence into the reactor 96.At its lower end the damper 102 possesses an operating arm 108 to whicha double acting hydraulic or pneumatic cylinder 110 is connected formoving the damper 102 between its open and closed positions.

The connecting pipe 96 at the upper end of the discharge stack 90 notonly directs hot gases from the stack 90 into the reactor 96, butfurther receives fumes from the fume hood 80 of the charging well 6 aswell. In this connection, the fume hood 80 is connected with the upperend of the discharge stack 90 through a duct 112 (FIG. 2) containing adamper 114.

The other discharge stack 92 (FIG. 2) extends upwardly from the roof 38of the sweat chamber 8 over which it is centered (FIG. 5--phantom linesin FIG. 3), its cross-sectional area being about equal to the combinedcross-sectional area of the two ports 36 in the side wall 16. Actually,the cross-sectional area of the two ports 36 should range between 1 and1.5 times the cross-sectional area of the stack 92 where the stack 92opens into the sweat chamber 8. The stack 92 likewise possesses a damperseat 116, an opening 118, and a refractory damper 120 which correspondin configuration and operation to the seat 98, opening 100 and damper102 for the stack 94, respectively. Moreover, the damper 120 is operatedby another double acting cylinder 122.

The two connecting pipes 94 lead into the base of the reactor 96 whichcontains (FIGS. 2 & 6) a cylindrical reaction chamber 124 and adisk-like separator 126 at the bottom of that chamber, that is betweenthe reaction chamber 124 and the ends of the connecting pipes 94. Inaddition the reactor 96 has two burners 128 which are directed generallytangentially into the reaction chamber 124 to produce a swirlingcombustion which further consumes volatiles that are derived primarilyfrom the charging well 6. The reaction chamber 124 of the reactor 96opens into a final stack 130 through which the hot and cleansed gasesare vented to the atmosphere.

OPERATION

During the normal operation of the furnace A, the bath B exists in amolten condition in the main chamber 2, the pumping well 4 and thecharging well 6 which are all interconnected below the upper surface ofthe bath B. Thus, the upper surface of the bath B is essentially at thesame elevation at all three locations, and that elevation should be nohigher than the lower margin of the sill 30 for the main chamber 2 andthe lower margins of the dross ramps 74 for the charging well 6.

The pump 64 within the pumping well 4 causes the molten aluminum to flowfrom the main chamber 2 to the pumping well 4 and thence into thecharging well 6, whereupon it returns to the main chamber 2, thiscirculation being made possible by submerged openings 60 and 100 in theside wall 18 and the opening 62 in the separating wall 56.

Molten aluminum is withdrawn from the furnace A from time to time at thetap 31 so as to provide molten aluminum for a casting, extruding or someother manufacturing operation.

To maintain the bath B in a molten condition, the burners 32 in the roof26 of the main chamber 2 operate to direct flames into the upper portionof the main chamber 2, that is, the portion that is above the bath B.The heat produced by the flames from the burners 32 is absorbed by thealuminum of the bath B as well as by the walls 16, 18 and 20, the doors22 and the roof 26 which line the main chamber 2 and indeed almosttotally enclose the upper portion of the main chamber 2 with surfaceareas. While these surface areas absorb heat, they also reradiate theheat to the bath B within the main chamber 2, and to achieve maximumreradiation, it is desirable to have the ports 36 as small as possible.On the other hand, at this time in the operation the refractory damper120 above the sweat chamber 8 is open, while the refractory damper 102above the main chamber 2 is closed (FIG. 2), and this causes the hotgases resulting from the combustion at the burners 32 to flow throughthe ports 36 in the side wall 16, whereupon they pass through the sweatchamber 8 and into the discharge stack 92 that leads away from thechamber 8. Thus, the ports 36 should be large enough to accommodate allof the flue gases from the main chamber 2, and hence their combinedcross-sectional size should be about as great as the cross-sectionalsize of the interior of the discharge stack 92. The sweat burners 50remain off at this point in the operation.

Light aluminum scrap, such as expended beverage cans, aluminum turnings,and aluminum sheet metal remaining from blanking operations, isintroduced into the furnace A at the charging well 6 by raising one ofthe doors 82 on the fume hood 80 and depositing the scrap in the moltenaluminum bath B within the charging well 6. Any lacquers, paint orhydrocarbons on the scrap immediately volatilize and pass upwardly intothe duct 112 that leads away from the fume hood 80. The duct 112 directsthese fumes into the nearby connecting pipe 94 which in turn directsthem into the reactor 96 where they enter the swirl in the reactionchamber 124 and are consumed by the flames discharged from the burners128 which produce that swirl. Oxidized aluminum together withcontaminants on the scrap produce a solid waste, known as dross, whichfloats on the surface of aluminum bath B within the charging well 6.From time to time the dross is removed by raising one of the doors 82for the fume hood 80 and raking the dross up over the dross ramp 74 atthat door.

Rarely is enough scrap available to supply the needs of the furnace A,and indeed the primary needs are supplied from aluminum ingots, usuallyreferred to as sows, which are placed in the sweat chamber 8 where theyrest upon the sweat hearth 34. Before opening the doors 42 to load thesweat chamber 8, the burners 32 for the main chamber 2 are shut off, andof course the burners 50 for the sweat chamber 8 are likewise off. Thenthe sows C are stacked on the sweat hearth 34 and the doors 42 areclosed.

The sows C are next preheated in the sweat chamber 8, and this isachieved by operating the furnace A in essentially the manner previouslydescribed. More specifically, the damper 102 in the stack 90 leadingfrom the main chamber 2 is left closed, while the damper 120 in thestack 92 leading from the sweat chamber 8 remains open. Moreover, theburners 32 in the main chamber 2 are reignited, and of course the heatwhich is produced maintains the bath B in a molten condition. The hotgases which develop pass through the ports 36 in the division wall 16and through the sweat chamber 8 where they elevate the temperature ofthe sows C. Indeed, the temperature of the sows C rises well above theboiling point of water, so that any water that may be trapped inshrinkage cracks possessed by such sows boils off and escapes throughthe discharge stack 92. Thus, in this mode of operation the heatproduced by the flames issuing from the burners 32 not only serves tomaintain the bath B in a molten condition, but further serves to preheatthe sows in order to purge them of all water.

Once the sows C reach the maximum temperature to which the gases fromthe main chamber 2 are capable of elevating them, they may be meltedwithin the sweat chamber 8, provided the main chamber 2 has enoughcapacity to accept the volume of aluminum contained within the sows C.To melt the sows C the burners 32 of the main chamber 2 are shut off andthe refractory dampers 102 and 120 are reversed, that is the damper 120in the stack 92 leading from the sweat chamber 8 is closed, while thedamper 102 in the stack 90 leading from the main chamber 2 is opened.The burners 50 for the sweat chamber 8 are energized, and they projectflames in the sweat chamber 8. These flames heat the sows C as well asthe surrounding walls 16, 20, 38, 40, and 42 which enclose the sweatchamber 8, and those walls of course radiate heat back to the sows C.Thus, maximum heat is extracted within the chamber 8 from the flamesissuing from the two burners 50, and that heat melts the sows C. Themolten aluminum flows across the sweat hearth 34 to the ports 36, andthence through the ports 36 into the bath B, thus adding to the aluminumin that bath B.

The hot gases produced by the flames that issue from the burners 50likewise pass out of the sweat chamber 8 through the ports 36 in thewall 16, and enter the upper portion of the main chamber 2 through whichthey pass as they flow toward and into the discharge stack 90 for thatchamber. In so doing they maintain the walls 16, 18, 20, 22, and 26 ofthe main chamber 2 at essentially their operating temperature and indeedmaintain the bath B in a molten condition. Thus, in this mode ofoperation the heat from the burners 50 likewise is utilized in twolocations, that is in the sweat chamber 8 where it melts the sows C andin the main chamber 2 where it maintains the bath B in a moltencondition.

Once the sows C are totally melted, the furnace is returned to itsnormal mode of operation. To this end, the burners 50 are shut down, thedampers 102 and 120 are again reverse so that the former is closed andthe latter is opened, and the burners 32 are reignited.

Furthermore, the presence of the side or division wall 16, whichseparates the main chamber 2 from the sweat chamber 8, enables workmento load sows C onto the sweat hearth 34 without being exposed to theintense heat of the bath B in the main chamber 2. The side wall 16further renders the furnace A safe to operate, because the sows C cannotbe accidentally pushed off of the sweat hearth 34 and into the moltenbath B.

While the construction and operation of the furnace has been describedin connection with aluminum metal, the furnace A may be used to providea molten supply of some other metal as well.

This invention is intended to cover all changes and modifications of theexample of the invention herein chosen for purposes of the disclosurewhich do not constitute departures from the spirit and scope of theinvention.

What is claimed is:
 1. A process for providing a source of molten metal,said process comprising: confining molten metal in a main chamber sothat a bath of the molten metal exists in the main chamber; directing aflame into the main chamber to heat the metal in the main chamber andmaintain it molten; placing rearwardly large solid pieces of metal in asweat chamber that is separated from the main chamber by a division wallwhich contains at least one port through which the sweat chambercommunicates with the main chamber, the total cross-sectional area ofthe port or ports being substantially less than the correspondingcross-sectional area of the chambers beyond either end of the ports;venting the main chamber through the sweat chamber and thence through avent that leads away from the sweat chamber so as to preheat the solidpieces of metal in the sweat chamber; thereafter directing a flame intothe sweat chamber to melt the solid pieces of metal in the sweat chamberallowing the molten metal in the sweat chamber to flow into the bathwithin the main chamber; and while the flame is directed into the sweatchamber, venting the sweat chamber through the port or ports, the mainchamber and another vent that leads away from the main chamber, so thatthe heat produced in the sweat chamber serves to maintain the bath inthe main chamber molten.
 2. The process according to claim 1 and furthercomprising maintaining some of the bath in a pumping well and a chargingwell located adjacent to the main chamber, the two wells being incommunication with each other and with the main chamber below thesurface of the bath, whereby the surface of the bath exists at the samelevel in the melting chamber and the pumping and charging wells; fromwithin the pumping well imparting movement to the molten metal of thebath so as to cause the molten metal to circulate through the pumpingand charging wells; and introducing relatively light scrap metal intothe bath at the charging well.
 3. The process according to claim 1wherein the lower surface of the sweat chamber is above the uppersurface of the bath in the main chamber, and the sweat chambercommunicates with the main chamber along the lower surface of the sweatchamber, whereby when metal is melted in the sweat chamber, it drains bygravity into the bath in the main chamber.
 4. The process according toclaim 3 wherein the total cross-sectional area of the port or ports isabout the same as the cross-sectional area of the vent that leads awayfrom the sweat chamber.
 5. The process according to claim 1 wherein thefloor of the sweat chamber is above the molten metal in the main chamberand the port in the division wall extends down to the floor of the sweatchamber so that molten metal in the sweat chamber in flowing into thebath in the main chamber passes through the port.
 6. A process forproviding a source of aluminum metal, said process comprising: confininga molten metal to a bath in a main chamber having a first vent extendedfrom it, there being a first damper in the first vent; placing solidpieces of the same metal in a sweat chamber which through at least oneopening is in communication with the main chamber, the totalcross-sectional area of the opening or openings being substantiallysmaller than the corresponding cross-sectional areas of the chambersimmediately beyond the openings, the sweat chamber further having asecond vent extended from it, there being a second damper in the secondvent; closing the first damper and opening the second damper; thereafterdirecting a flame into the main chamber, with the flame producing enoughheat to maintain the bath within the main chamber in a molten condition,whereby the heated gases produced by the flame pass through the openingbetween the two chambers and into the sweat chamber where they preheatthe solid pieces of metal in the sweat chamber and then leave the sweatchamber through the second vent; after the solid pieces of metal in thesweat chamber are preheated, reversing the dampers so that the firstdamper is open and the second damper is closed; after the dampers arereversed, directing a flame into the sweat chamber with the flameproducing enough heat to melt the preheated solid pieces of metal,whereby the metal melts and the heated gases produced by the flames inthe second chamber flow through the opening between the chambers andinto the main chamber through which they pass and are thereafterexhausted through the first vent, so that the gases from sweat chambersupply heat to the bath in the main chamber; and directing molten metalfrom the sweat chamber into the bath of molten metal in the mainchamber.
 7. The process according to claim 6 wherein the flame directedinto the main chamber is extinguished before the flame is directed intothe sweat chamber, and vice-versa.
 8. The process according to claim 7wherein the sweat chamber has a floor which is at an elevation higherthan the upper surface of the bath in the main chamber and the sweatchamber communicates with the main chamber at the floor of the sweatchamber so that the molten metal produced in the sweat chamber drainsinto the main chamber.
 9. The process according to claim 7 wherein theopening between the two chambers has a total cross-sectional area whichis between 1 and 1.5 times the cross-sectional area of the second vent.10. A furnace for holding a supply of molten metal, said furnacecomprising: first walls enclosing a main chamber for containing a bathof the molten metal; first burners directed into the main chamber abovethe surface of the bath therein; a first discharge stack opening at itslower end into the main chamber; a first damper in the first stack andbeing movable between open and closed positions with respect to thefirst stack; second walls which in combination with at least one of thefirst walls enclose a sweat chamber, so that said one first wall of themain chamber constitutes a division wall that separates the main chamberfrom the sweat chamber, another of the walls of the sweat chamber beinga sweat hearth on which relatively large pieces of metal are supportedin the sweat chamber, the sweat hearth being at an elevation higher thanthe upper surface of the bath in the main chamber, the division wallthat separates the main and sweat chambers containing at least one portfor enabling hot gases produced by the first burners in the main chamberto flow into the sweat chamber and the total cross-sectional area of theport or ports being substantially smaller than the correspondingcross-sectional area of either chamber along the division wall, thedivision wall further permitting molten metal on the sweat hearth toflow into the bath in the main chamber; second burners directed into thesweat chamber for elevating the temperature of metal pieces placedtherein high enough to melt those metal pieces; a second discharge stackopening at its lower end into the sweat chamber; and a second damper inthe second stack and being movable between open and closed positionswith the respect to the second stack.
 11. A furnace according to claim10 and further comprising means enclosing a pumping well whichcommunicates with the main chamber below the surface of the bath andmeans enclosing a charging well which communicates with the pumping welland the main chamber below the surface of the bath, whereby the surfaceof the bath exists at the same elevation in the main chamber, pumpingwell and charging well, and a pump in the pumping well for circulatingmolten metal from the main chamber through the pumping well and chargingwell.
 12. A furnace for providing a supply of molten metal, said furnacecomprising: walls enclosing a main chamber for containing a bath of themolten metal, one of the walls of the main chamber being an upright sidewall; first heating means for heating and elevating the temperature ofthe main chamber; walls enclosing a sweat chamber that communicates withthe main chamber such that molten metal in the sweat chamber will flowinto the main chamber, one of the walls of the sweat chamber being asweat hearth on which relatively large pieces of the metal are placed,the sweat hearth being located at an elevation higher than the uppersurface of the bath in the main chamber and generally sloping downwardlytoward that bath, another of the walls of the sweat chamber beingvertical and also serving as the upright side wall of the main chamber,whereby the upright side wall separates the main and sweat chambers, theupright side wall containing ports through which the sweat chambercommunicates with the melting chamber, the ports being located lowenough to enable molten metal to drain from the sweat hearth into thebath in the melting chamber; second heating means for heating andelevating the temperature of the sweat chamber; a first discharge ventopening into the main chamber and leading away from that chamber; afirst damper in the first vent, the first damper being movable betweenopen and closed positions with respect to the first vent; a seconddischarge vent opening into the sweat chamber and leading away from thatchamber; and a second damper in the second vent, the second damper beingmovable between open and closed positions with respect to the secondvent.
 13. A furnace according to claim 12 wherein the first heatingmeans comprises at least one burner in a wall of the melting chamberabove the molten metal therein, and the second burner heating comprisesat least one burner in a wall of the sweat chamber.
 14. A furnaceaccording to claim 12 wherein the combined cross-sectional size of theports in the side wall ranges between 1 and 1.5 times thecross-sectional size of the second vent.
 15. A furnace according toclaim 12 wherein one of the walls enclosing the sweat chamber is a doorwhich may be moved to expose the sweat chamber so that pieces of metalmay be loaded into it.
 16. A furnace according to claim 12 and furthercomprising means enclosing a pumping well which communicates with themain chamber below the surface of the bath; means enclosing a chargingwell which communicates with pumping well and the main chamber below thesurface of the bath, so that the surface of the bath exists at the sameelevation in the melting chamber, the pumping well, and the chargingwell; and a pump in the pumping well for causing molten metal from thebath in the main chamber to circulate through the pumping well andcharging well.
 17. A furnace according to claim 16 and furthercomprising a hood over the charging well and a duct leading from thehood and communicating with one of the vents so that fumes from thecharging well are directed into one of the vents.
 18. A furnace forholding a supply of molten metal, said furnace comprising: first wallsenclosing a main chamber for containing a bath of the molten metal;first burners directed into the main chamber above the surface of thebath therein; a first discharge stack opening at its lower end into themain chamber; a first damper in the first stack and being movablebetween open and closed positions with respect to the first stack;second walls which in combination with at least one of the first wallsenclose a sweat chamber, so that said one first wall of the main chamberconstitutes a division wall that separates the main chamber from thesweat chamber, another of the walls of the sweat chamber being a sweathearth on which relatively large pieces of metal are supported in thesweat chamber, the sweat hearth being at an elevation higher than theupper surface of the bath in the main chamber, the division wall thatseparates the main and sweat chambers containing at least one port forenabling hot gases produced by the first burners in the main chamber toflow into the sweat chamber; second burners directed into the sweatchamber for elevating the temperature of metal pieces placed thereinhigh enough to melt those metal pieces; a second discharge stack openingat its lower end into the sweat chamber; and a second damper in thesecond stack and being movable between open and closed positions withrespect to the second stack, the total cross-sectional area of the portin the division wall ranging between about 1 and 1.5 times thecross-sectional area of the second stack where the second stack opensinto the sweat chamber.
 19. A furnace according to claim 18 wherein theport in the division wall extends down to the sweat hearth so thatmolten metal on the sweat hearth drains through the port and into thebath in the main chamber.
 20. A furnace for holding a supply of moltenmetal, said furnace comprising: first walls enclosing a main chamber forcontaining a bath of the molten metal; first burners directed into themain chamber above the surface of the bath therein; a first dischargestack opening at its lower end into the main chamber; a first damper inthe first stack and being movable between open and closed positions withrespect to the first stack; second walls which in combination with atleast one of the first walls enclose a sweat chamber, so that said onefirst wall of the main chamber constitutes a division wall thatseparates the main chamber from the sweat chamber, another of the wallsof the sweat chamber being a sweat hearth on which relatively largepieces of metal are supported in the sweat chamber, the sweat hearthbeing at an elevation higher than the upper surface of the bath in themain chamber, the division wall that separates the main and sweatchambers containing at least one port for enabling hot gases produced bythe first burners in the main chamber to flow into the sweat chamber,the port in the division wall extending down to the sweat hearth so thatmolten metal on the sweat hearth drains through the port and into thebath in the main chamber; second burners directed into the sweat chamberfor elevating the temperature of metal pieces placed therein high enoughto melt those pieces; a second discharge stack opening at its lower endinto the sweat chamber; and a second damper in the second stack andbeing movable between open and closed positions with respect to thesecond stack.